This invention relates to a steam condensing apparatus including a container containing coolant and a guide pipe with its distal end extended into the coolant to lead steam of the coolant into the coolant.
The steam condensing apparatus of this type is generally known. Some of plants that handle steam conventionally use apparatus to operate to lead high-pressure steam discharged from a safety valve into coolant, thereby condensing the steam, when the steam pressure is raised to an excessive level. Typical examples of such plants include boiling water reactor plants. In a conventional boiling water reactor, a core is set in a pressure container which is housed in a housing container. Light water is fed into the pressure container, where it is converted into steam by heat generated from the core, and taken out to be used for driving a turbine, for example. A safety valve to operate when the steam pressure inside the pressure container exceeds a given value is attached to the steam exhaust port of the pressure container. Steam discharged through the safety valve is ejected into coolant or light water in a primary containment vessel used as a coolant container by means of a guide pipe, and condensed.
The aforesaid apparatus for condensing the exhausted steam, which is used as an essential apparatus to constitute a plant, still leaves room for improvement. Such problems will now be described in connection with the apparatus for the aforementioned boiling water reactor plant.
Normally, coolant penetrates into the lower portion of the interior of the guide pipe to substantially the same level as the coolant in the primary containment vessel, thereby forming a column of coolant. In this state, when high-pressure steam from the safety valve flows into the guide pipe, uncondensable gas (hereinafter referred to simply as gas) introduced into the guide pipe is first compressed to force out the coolant from the guide pipe, and then ejected into the coolant. Thereafter, the steam discharged from the safety valve is ejected into the coolant. The gas is compressed first because the coolant will not be able to move quickly due to the inertia of the coolant column and the flow resistance even if gas pressure is applied to the coolant.
Conventionally the guide pipe has two branch pipes of equal length, at each distal end of which is formed a nozzle of same diameter. The gas ejected from the nozzle forms two high pressure bubbles. First expanding in the coolant, the gas bubbles, substantially simultaneously, repeat contraction caused by overexpansion and expansion caused by overcontraction, rise in the coolant while generating oscillatory pressure fluctuations, and leave the surface of the coolant. When the pressure fluctuations reach the inside (wall) of the primary containment vessel, dynamic load is applied to the vessel. Such dynamic load will be hereinafter referred to as load created by bubble oscillation or first dynamic load.
Following the gas ejection from each nozzle, high-pressure steam is ejected into the coolant to form a steam region therein. The higher the flow rate of the ejected steam, the greater the distance covered by the steam region will be. Further, the wider the exhaust nozzle of the guide pipe, the thicker the steam region will be. The configuration of the steam region should be maintained substantially constant as long as the flow rate of steam supplied to the region is balanced with the condensation speed of the steam. Actually, however, it is very difficult to maintain such balance, so that the steam region will repeat expansion and contraction. The expansion and contraction of each region causes pressure fluctuations in the coolant in the primary containment vessel. The pressure fluctuations substantially simultaneously reach the inside of the primary containment vessel and applies dynamic load to the vessel. This dynamic load generated by the steam regions will be hereinafter referred to as second dynamic load. The ejection of the coolant column, gas and steam into the coolant are made successively through each exhaust nozzle attached to the guide pipe.
In the prior art apparatus, as described above, the first and second dynamic loads generated by the gas and steam ejected from two nozzles of a guide pipe are applied to the primary containment vessel in discharging high-pressure steam into the coolant. Accordingly, it is necessary to manufacture the primary containment vessel which has enough mechanical strength to resist those dynamic loads. Naturally, such mechanical strength must be in compliance with the safety standards applicable to reactor plants. If the dynamic loads are great, therefore, the design of the primary containment vessel will become difficult to cause inevitable increase in size and cost of the apparatus. Thus, there is an increasing demand for the development of steam condensing apparatus capable of reducing those dynamic loads.